Contamination of subsurface environments by fluids that are immiscible with water has occurred routinely in the United States and industrialized countries around the world over the last 40 years. Mercer and Cohen (1990). Such fluids are often termed non-aqueous phase liquids (NAPLs) in general, or LNAPLs when less dense or DNAPLs when more dense than the groundwater present in the subsurface. Typical LNAPLs are petroleum products (e.g., gasoline, diesel fuel, jet fuels, heating oils), and typical DNAPLs are chlorinated solvents (e.g., trichloroethylene, tetrachloroethylene, dichloroethanes). Schwille (1988); Bartow and Davenport (1990).
Species present in such NAPL phases can solubilize to the aqueous phase, volatilize to the gas phase, or sorb to the solid phase present in the subsurface. Environmental concerns result when such species are linked to human or ecological health concerns and become present in sufficient quantities in a mobile aqueous or gas phase. Such health concerns are associated with common constituents of NAPLs found routinely in the environment, such as trichloroethylene and benzene.
Once released into the subsurface, NAPLs migrate and typically reach a stable, immobile state within a relatively short time scale (hours to days) after the source is removed. Immobile NAPLs can remain in the subsurface over time scales that can range from months to decades or longer under natural conditions, because they are comprised of species that are only sparingly soluble in water. Miller, Poirier-McNeill et al. (1990). As a result, NAPLs are considered a long-term source of groundwater contamination. Mayer and Miller (1996).
DNAPLs tend to be an even more significant problem than LNAPLs because of the following characteristics:
(1) they were routinely used in industrial practices, spilled, and intentionally disposed of in the subsurface in the United States, starting in the 1960's and continuing for two decades; PA1 (2) they often migrate larger distances than LNAPLs; PA1 (3) they often penetrate the water table; PA1 (4) they can form pools contained by low-permeability materials; PA1 (5) they are often comprised of species that tend to degrade slowly in many systems; PA1 (6) they are extremely difficult to locate and remove; and PA1 (7) they contain species that are typically regulated at low concentrations in drinking water (e.g., 5 .mu.g/L).
Because of the above characteristics, remediation of subsurface contamination resulting from DNAPLs is a frequently encountered problem, which has proven to be extremely difficult. Methods that have been used to remove such contamination include: pump-and-treat, cosolvent flushing, surfactant flushing, steam injection, and in situ biodegradation. To varying extents, all of these strategies have been implemented at the laboratory and pilot or full field scale. Each of these methods results in the removal of some solute mass from a system contaminated with a DNAPL, but the rate at which the removal occurs and the expense involved with this standard set of methods leaves the problem of DNAPL remediation unsolved. The advantages and disadvantages of the common set of DNAPL removal and containment strategies are described in detail as follows.